Field of the Invention
The present invention generally relates to mobile communications technologies, and more particularly, to Rank Information (RI) feedback.
Description of the Prior Art
In a modern mobile communications system such as Long Term Evolution (LTE), a base station determines a transmission format, a transmission block size, a modulation and coding scheme, and a Multiple Input Multiple Output (MIMO) transmission mode to be used in a downlink (DL) and an uplink (UL). To perform such determination for the DL, the base station needs information about the performance of a current DL channel from a User Equipment (UE), and said information is generally referred to as Channel State Information (CSI).
Frequency Division Duplexing (FDD) DL coordinated multi-point (CoMP) transmission has potential in improving the coverage area, cell edge throughput, and/or spectral efficiency. In the CoMP transmission, multiple CSI reference signal resources (CSI-RS-resources) coordinate with each other to transmit data of one or more UEs. Because of the high space division multiplexing gain and/or better channel quality of high rank transmission, certain UEs may prefer high rank transmission to single rank transmission, especially in dynamic point selection (DPS) or dynamic point blanking (DPB). Therefore, the high rank transmission needs to be supported in the CoMP; however, a conventional RI report is no longer applicable.